For years, the measurement of oil viscosity in a lubricating system has been relegated to an intermittent and static off-line determination. There has been a long felt need for a means of monitoring the viscosity of lubricating fluids real time and on-line. There are commercially available viscometers, but it is difficult to achieve consistent and accurate on-line viscosity measurements using these viscometers. Many of the commercially available viscometers are not well suited for on-line process control, because they are very sensitive to contaminants and temperature gradients in both the lubricating fluids and the viscometer itself. In a process environment, it is very difficult to maintain lubricating fluids free of contaminants and to accurately control the temperature of the lubricating fluid to a fixed temperature.
With conventional methods and systems for measuring viscosity, temperature control is crucial to achieving accurate results. The significance of temperature control for achieving accurate viscosity measurements with conventional viscometers has been confirmed by laboratory experiments. For example, for a sample of lubricating fluid held in a constant temperature bath for an extended period of time, the measured viscosity of the sample of lubricating fluid can change 5%-10% due to effective temperature fluctuations at the measurement point even without significant change in the bulk temperature of the sample of lubricating fluid. Consistent viscosity measurements can only be achieved if the sample of lubricating fluid, the sensor head of the viscometer, and the sensor stem of viscometer are held at the same constant temperature.
The problem of temperature control is further complicated if viscosity is estimated by combining data from several pieces of equipment spread across a system, rather than being measured directly with a viscometer. Most commonly, viscosity is estimated by measuring flow across a measured pressure differential and using the Hagen-Poiseuille equation to determine viscosity. As noted above, this type of method of estimating viscosity further complicates the problem of temperature control. The temperature of the lubricating fluid usually needs to be controlled at all the measurement points (e.g., at the locations of any flowmeters and pressure sensors). Such temperature control can be very difficult, particularly, if the measurement points are located in different parts of a process system having different operating temperatures. Variations in the temperatures at the various measurement points can also vary depending on a number of conditions, such as time of day, season, equipment status, and the like.
A data simulation, as illustrated by FIG. 1, quantitatively demonstrates the magnitude of the impact that temperature has on viscosity measurement: for this particular fluid, a change as small as one degree Celsius can change viscosity by about 4%-7%. As shown in FIG. 1, for an ISO 220 industrial lubricant oil, the kinematic viscosity measured in centistokes at 40 degrees Celsius (“cSt @ 40° C.”) is about 5.7% higher than at 41° C. This difference does not even account for any random noise error that may be contained in the viscosity measurement. To achieve viscosity measurement biases lower than 5% with conventional methods and systems, it may be necessary to accurately control measurement temperatures to within 1° C., or better. The sensitivity of viscosity to temperature depends on the type and composition of fluid being tested.
In light of problems with prior-art methods and systems of determining viscosity on-line, there is a need for systems and methods of determining viscosity on-line that compensate for temperature differences in the system. Accordingly, this invention satisfies that need.